To fit different people, an automobile usually has a seat adjuster assembly which comprises a motor driving a spindle shaft via a gearbox. The spindle shaft has outer threads engaging with inner threads of a nut contained in a tube which in turn is connected to a linkage connected respectively to a vertically movable upper part of a vehicle seat and a non-vertically movable lower part of the vehicle seat to cause the seat to move up or down under the control of a passenger occupying that seat to obtain a comfortable position. The lower part of the vehicle seat is usually slidingly mounted in guide rails fixed to the vehicle frame so that the seat can be positioned fore and aft.
Usually, the gearbox comprises a worm gear mounted on the output shaft of the motor and a spur gear meshing with the worm gear. The spur gear is made of plastic and directly molded to the spindle shaft. However, the plastic spur gear cannot withstand a large axial force by itself and is easily destroyed by a large axial force imposed on the spindle shaft, as would occur in the event of a crash or accident.